The present invention relates generally to semiconductor fabrication and, more particularly, to an apparatus and method for improving substrate bake uniformity.
As semiconductor device manufacturers continue to produce smaller devices, the requirements of photomasks used in the fabrication of these devices continue to tighten. Photomasks, also known as reticles or masks, are typically high-purity quartz or glass plates used to transfer circuit images onto semiconductor wafers such as silicon. As devices have gotten smaller, the circuit images on the mask have become smaller and more complex. Consequently, the quality of the mask has become one of the most crucial elements in establishing a robust and reliable semiconductor fabrication process.
One of the most challenging requirements in producing masks is controlling critical dimension (CD) uniformity. CD uniformity may be defined as the statistical results of measurements of similar sized lines and spaces that are written across a mask. CD uniformity is generally specified as a range, three sigma or both.
Many participants in the electronics industry are driven to reduce CD uniformity error. These participants include manufacturers of lithography tools, processing tools, photoresist, x-ray resist and tools, as well as manufacturers of deep ultra-violet resist and tools, phase shift masks and CD measurement tools.
Efforts to reduce CD uniformity error may also be found in photomask blanks. Blanks typically consist of synthetic quartz substrates coated with chrome and a selected resist. Circuit patterns are typically written directly on blanks in a mask shop.
One of the most common photomask blanks is the 6025-Chrome-895 blank. The 6025 blank is typically a six inch (6.0xe2x80x3) square, one quarter inch (0.25xe2x80x3) thick, synthetic quartz substrate supplied by Shin Etsu, Asahi and Chi Chi Bu glass manufacturers. Chrome is typically applied to the blank in a manner similar to that described in U.S. Pat. No. 5,230,971. The optical resist that is applied to the substrate may be identified by xe2x80x98895.xe2x80x99 This resist is a photo-optical polymer, typically in liquid form, and can be obtained from various suppliers including Arch Chemicals. The resist is typically spin coated onto the chrome/quartz substrate and subsequently baked.
In an effort to understand CD uniformity error in blanks, a study of temperature uniformity across the surface of a baking 6025 blank was performed. The temperature study found the substrate surface to have a seven degree Celsius (7xc2x0 C.) range. One problem in substrate surface temperature uniformity was determined to be related to the distance between the heated surface used to bake the substrate and the surface of the substrate, also known as the proximity gap.
Previously, controlling the proximity gap was accomplished by stacking layers of thin tape, typically Kapton tape, together to obtain a desired height. The use of tape presents a variety of problems for precise control of proximity gaps. For example, as a thin plastic, the tape is subject to deformation during application. Furthermore, it is common for the tape to thin at baking temperatures over its useful life. In addition, exact placement of the tape is necessary and the tape cannot generally be positioned as accurately as needed.
In addition to problems associated with controlling the proximity gap, a series of problems are associated with typical hot plates commonly used to bake substrates. First, a typical hot plate is made from a block of aluminum. Due to the softness of aluminum, the heated surface or surfaces of an aluminum hot plate can generally not be machined flat below two hundred microns (200 xcexcm). The softness of aluminum and most frequently used grinding techniques prevent obtaining tighter tolerances for flatness or smoothness. Second, since the substrates typically being heated are round and the hot plate surface is typically square, more heat is commonly absorbed by air surrounding the corners of the hot plate than in the center. This causes a lowering of the temperature at the edges of the substrate which effects CD uniformity in the blank.
In an attempt to overcome these and other disadvantages, hot plates have been designed which use radial heater coils having individual temperature controllers to heat each section of the hot plate. For such an apparatus to be effective, all of the individual temperature controllers must be properly calibrated and working. A failure in a single temperature controller would typically be compensated for by another temperature controller thereby leading to undetectable temperature errors.
In accordance with teachings of the present disclosure, a method, system and apparatus for improving temperature uniformity in a substrate during baking are provided.
In one aspect, a method for heating a substrate is provided. The method preferably includes maintaining a hot plate having a recess defined by a bottom and at least one wall disposed in its first surface. Further, the method preferably also includes applying heat to a second surface of the hot plate and positioning the substrate above the recess, proximate the first surface of the hot plate, such that respective first and second surfaces of the substrate are maintained generally parallel to the first surface of the hot plate and to the bottom of the recess. Also, an outer edge of the first surface of the substrate is preferably maintained proximate the wall defining the recess.
In another aspect, the method for baking a substrate may include applying heat to a hot plate, positioning a first surface of the substrate a distance from the hot plate and varying the distance between the first surface of the substrate and the hot plate to maintain approximately uniform application of heat to the first surface of the substrate and a more uniform temperature across the first surface of the substrate.
In a further aspect, an apparatus for baking a substrate is provided. The apparatus preferably includes a hot plate having a recess disposed on its first surface. The recess may be defined by at least one wall and preferably has an approximately uniform depth. Further, the recess is preferably sized such that at least a portion of an outer edge of a substrate contacts a portion of the first surface of the plate when the substrate is positioned to cover the recess.
In yet another embodiment, a system for baking a substrate is provided. The system preferably includes a hot plate having a first recess with an approximately uniform depth disposed in a first surface of the hot plate. The recess is preferably defined by a bottom and at least one wall. The hot plate included in the system preferably also includes a second recess having an approximately uniform depth disposed in the second surface of the hot plate. Similar to the first recess, the second recess is preferably defined by a bottom and at least one wall. In part to facilitate baking, the system also preferably includes a heating source preferably coupled to the second recess. In the system, the hot plate is preferably operable to maintain the substrate proximate its first surface such that the first recess is covered and a portion of the substrate""s first surface is in contact with the first surface of the hot plate. The hot plate is preferably further operable to establish an approximately uniform temperature in the substrate during baking.
One technical advantage provided by the present invention is a reduction in critical dimension uniformity error due to non-uniform bakes.
An additional technical advantage provided by the present invention is a reduction in backside defects which can occur on blanks due to Kapton tape wear.
A further technical advantage provided by the present invention is an ability to manufacture more critical mask layers with improved blanks.